Filter beds formed from one or more layers of filter media have been employed in a variety of known filters for filtering water or wastewater to remove impurities from liquids. For example, filter beds of granular media have been used in upflow filters, downflow filters as well as other type of filters including bi-flow filters. After the filter has been operating for a while, it is necessary to wash the filter bed to remove the impurities trapped in the filter beds during the filtration mode. Various methods have been used to wash the filter bed including but not limited to the steps of: (i) liquid only wash; (ii) air only wash; (iii) liquid and air concurrently; (iv) liquid only followed by air only; (v) air only followed by liquid only; and, (vi) liquid and air concurrently followed by liquid only.
It is important that the washing fluid is thoroughly distributed through the filter bed during the washing mode in order to remove the impurities trapped in the filter bed during operation of the filter in the filtration mode. Various underdrains and flumes have been used in an attempt to distribute the washing fluids uniformly throughout the filter beds. One significant problem encountered by prior art devices is their inability to accommodate changes in the washing procedure including changes in the flow rates and changes in the washing fluids. For example, a washing procedure that employs simultaneous liquid and air washing fluids is highly susceptible to mal-distribution of the washing fluid through the filter bed. Typically, in systems using this type of washing procedure, the filter bed is disposed above the underdrain. The underdrain often consists of a plurality of underdrain laterals placed in a side-by-side fashion. The underdrain laterals direct gas and liquid through the filter bed during the washing mode. The underdrain laterals are in fluid communication with a flume. The flume receives the washing fluids (i.e., gas and liquid) from their source and directs these fluids to the underdrain laterals. The gas/liquid interface in these flumes is often low, i.e., close to the bottom of the flume. This is undesirable as it limits the area available for the washing liquid resulting in relatively high liquid washing flow velocities down the length of the flume which in turn causes mal-distribution of the washing fluids to the underdrain and ultimately the filter bed.
One proposed solution to the low gas/liquid interface problem in the flume is to provide a flume with a bottom that is lower than the bottom of the filter bed. An example of this type of construction is shown in FIG. 3 of U.S. Pat. No. 6,312,611. Alternatively, separate members have been provided for conveying and distributing liquid and gas separately in an attempt to overcome the low gas/liquid interface problem. Examples of these types of devices are illustrated in FIGS. 4 through 7 of U.S. Pat. No. 6,312,611.
Another proposed solution to the low gas/liquid interface problem has been to provide at least one flume liquid metering orifice (i.e., a closed perimeter opening) in a particularly shaped baffle or stand-pipe. Examples of these structures are shown in FIGS. 10 through 20 of U.S. Pat. No. 6,312,611. The orifices, i.e., the closed perimeter openings, are preformed in baffle or the stand-pipe. FIG. 1 of the subject specification illustrates the orifice (i.e., a closed perimeter opening) design in a baffle wall. Specifically, an enclosed chamber/flume 2 includes a baffle wall 4 having an orifice 6. Air wall sleeve 8 and water wall sleeve 10 connect the flume 2 to the underdrain 12.
Forming orifices of the type illustrated in FIG. 1 of the subject specification normally requires expensive formwork to accommodate the air and water wall sleeves that are cast in place. To be cost effective, the wall sleeves are made using commercially available sizes, which limits the available orifice sizes. Hence, most designs are not optimal but rather less efficient designs due to the compromise between technical considerations and commercially available materials.
The orifice design disclosed and claimed in U.S. Pat. No. 6,312,611 is also inferior due to its inability to tailor the headloss characteristics over a range of flows. This is particularly troublesome when the washing procedure includes a first step of concurrent liquid and gas followed by liquid only. The velocity of the washing fluid in the first step is typically in the range of 5 to 10 gpm/sq.ft. However, the velocity of the washing fluid in the second step is in the range of 15 to 25 gpm/sq.ft.
The increase in the flow rate of the washing fluid in the second step dramatically impacts the headloss across the orifice. This is especially true where the design is such that the fluid can only pass through the orifice. U.S. Pat. No. 6,312,611 discloses several such designs including but not limited to FIGS. 15, 16, and 20. The impact of the increase in flow rate is seen by analyzing the well known flow vs. headloss relationship of Q=cA√{square root over (2 gh)} where Q=flow, c=the orifice discharge coefficient, A=the cross-sectional area of the orifice, g=gravitational constant, and h=headloss across the orifice. Notably, the headloss across the orifice increases proportional to the square of the flow rate. If the backwash flow rate is increased from 5 gpm/sq.ft. to 25 gpm/sq.ft., the headloss across the orifice will increase by
      (          25      5        )    2or 25 times the headloss across the orifice at a flow rate of 5 gpm/sq.ft. This dramatic increase in headloss creates very difficult flow control issues. It is very important that the change in flow from the concurrent gas/liquid rate to the liquid only rate be made very gradual and controlled. Poor flow control will likely result in disruption of the gas/liquid interface within the flume, mal-distribution of the washing fluids, and the undesirable release of uncontrolled air into the underdrain. These conditions will likely cause structural failures of the underdrain system, disruption of the support gravel, and poor performance. As is readily evident from the above discussion, the orifice design is inflexible from the design stand-point and the application stand point as the design is limited to commercially available sizes and is unable to accommodate changes in the washing procedure.